Crotch to bottom of the leg with diaper on. Personally I think it should be center of the crotch which would include gusset measurements. Because the width of the gusset does affect the way the legs will fit. When I first started on DS I remember one mama complaining she had ordered 2 pairs of longies from 2 different knitters. One pair fit perfectly. One did not. She said when she measured the inseam both were correct. She couldn't figure out why the discrepancy in fit. Now having made my own and seen how they fit my child I think her issues were probably due to the gusset width rather than inseam measurements.